This invention relates to a method for treating titanium aluminide structures so as to provide improved high temperature properties.
Considerable research work has been directed toward the development of materials suitable for use in fabricating jet engine parts and other aircraft and aerospace components. A suitable material must be light weight and resistant to oxidation while having high creep, fatigue and tensile strength. Examples of such components include compressor blades, engine casings, heat panels and exhaust gas ducts.
A leading candidate material for these applications is titanium, particularly titanium alloys. However, the use of titanium alloy components is limited by the alloy's high reactivity to oxygen and the formation of an unstable oxide layer, at temperatures above about 1000.degree. F. (538.degree. C.), resulting in a degradation of mechanical properties.
Titanium aluminides, both as monolithic materials and as matrices for fiber-reinforced composites, have received considerable attention due to their potential superior high temperature properties together with their relatively low density. However, one obstacle to full use in both advanced gas turbine engine applications and advanced hypersonic structures is their low resistance to environmental degradation at higher temperatures. For example, it has been shown for alpha-2 titanium aluminides that exposure in an air environment to temperatures exceeding 1200.degree. F. (650.degree. C.) can result in significant embrittlement of the material surface which, in turn, leads to surface cracking under an applied load (in the case of a monolithic material) or after thermal cycling (in the case of a composite material). This surface embrittlement is the result of (1) the formation of a tenacious brittle oxide layer and (2) solid state diffusion of oxygen into the aluminide substrate, adversely affecting its ductility. Titanium aluminide alloys typically contain niobium as a beta stabilizing element and for enhanced oxidation resistance. It has been shown that local depletion of niobium from the exposed surface, forming a niobium oxide, results in the formation of a very brittle alpha or alpha-2 layer immediately below the oxide. When such material is loaded, particularly under cyclic, mechanical and thermal conditions, the brittle oxide cracks, providing a notch effect on the matrix as well as a preferred path for continued oxygen penetration and local embrittlement. Inasmuch as titanium aluminides are extremely notch sensitive, the result is a pronounced degradation in mechanical properties, both monotonic as well as cyclic. Brittle coatings, such as those which form a titanium aluminide coating based on TiAl.sub.3, have proven to be ineffective in protecting these materials because the coating cracks early in loading and provides preferred sites for environmental attack.
Fujishiro et al, U.S. Pat. No. 4,181,590, disclose that components fabricated from titanium alloys can be rendered resistant to oxidation by implanting ions of a noble metal or noble metal alloy into the substrate and thereafter continuing impingement of the metal ions onto the substrate to form a film or coating thereon. This method is quite effective for the alpha, alpha-beta and beta titanium alloys. However, the titanium aluminide alloys are generally more prone to surface cracking under an applied load or after thermal cycling, thus leading to cracking of any oxidation resistant coating. What is desired is a structure which provides oxidation resistance to the titanium aluminide component and which is resistant to cracking under an applied load or thermal cycling.
Thus it is an object of the present invention to provide a method for protecting a titanium aluminide substrate against environmental degradation.
Other objects and advantages of the invention will be apparent to those skilled in the art.